Photovoltaic cells are commonly used to convert light energy into electric energy (being also known as solar cells for use with the sun light). The most common type of solar cell is based on a semiconductor substrate (for example, made of silicon), wherein a PN junction is formed between a front surface and a back surface thereof; the sun light that is absorbed by the front surface of the substrate generates electric charges (i.e., electron-hole pairs), which supply a corresponding current to an external load.
Each solar cell generally has a front contact terminal on the front surface and a back contact terminal on the back surface for its coupling to the external load. The back contact terminal may extend throughout the whole back surface (since it is usually not reached by the sun light), so that it may be relatively thin. Conversely, the front contact terminal is typically maintained as small as possible, in order to limit the obscuring of the front surface to the sun light (for example, in the form of a grid with narrow contact strips); therefore, the front contact terminal is typically relatively thick (to reduce its resistance along the contact strips on the front surface).
A problem of each solar cell known in the art is the difficulty of maintaining the contact terminals fixed on the substrate, especially for the front contact terminal because of its small size and high thickness. Indeed, even a slight loss of adhesion of each contact terminal involves a non-uniformity or instability of its contact resistance, thereby causing a current concentration on the rest of the contact terminal; as a result, the contact terminal heats up with its progressive loss of adhesion. All of the above has a detrimental effect on the efficiency of the solar cell.
For this purpose, several techniques have been proposed to improve the adhesion of the front contact terminal (and of the back contact terminal as well) on the substrate.
For example, a known technique is based on applying a metal paste (for example, by a screen-printing process), and then performing a firing process (so as to anchor the metal paste on the substrate). However, the firing process requires the application of very high temperatures (of the order of 400-750° C.), which induce mechanical stresses on the solar cell (because of the different thermal expansion coefficients of its materials). Therefore, the solar cell is maintained relatively thick (for example, with a thickness of at least 150-200 μm), in order to sustain these mechanical stresses without cracking. The application of the metal paste also involves high production costs. Moreover, the metal paste provides a relatively high resistance of the contact terminals (which adversely affect the efficiency of the solar cell).
Another known technique is instead based on forming grooves on the front surface of the substrate (for example, by a laser ablation process), and then depositing a metal layer into them; these groves are relatively deep (for example, 3-60 μm), so that the front contact terminal so obtained is buried (at least partially) into the substrate (thereby remaining mechanically anchored to it). However, the groves weaken the mechanical structure of the solar cell. Therefore, as above the solar cell is maintained relatively thick (in order to avoid its cracking).
Porous silicon is also used in the production of the solar cells to form an antireflection coating (ARC) on the font surface of the substrate.
For example, Vinod et al., “The ohmic properties and current-voltage characteristics of the screen-printed silicon solar cells with porous silicon surface”, Solid State Communications, Pergamon, GB LNKD—DOI:10.1016/J.SSC.2009.02.019, vol. 149, no. 23-24, pages 957-961, XP026098082 ISSN: 0038-1098 (the entire disclosure of which is herein incorporated by reference) indicates that the solar cells may be produced forming the contact terminals by a screen printing step of an Ag paste followed by its firing (at 725° C.); the porous silicon is then formed by electrochemical etching on the n+-Si surface (in most cases without any protective cover of the Ag contacts). Alternatively (in order to avoid corresponding problems), the same document also indicates that the porous silicon may be formed first followed by the formation of the Ag contacts thereon; a firing step at 700-825° C. following by an annealing step at 450° C. are then performed to facilitate the formation of an ohmic contact between Ag and n+-Si (by driving molten glass frit contained in the Ag contacts nearly completely to pierce through the entire thickness of the porous silicon layer, thereby creating spike-like direct Ag—Si interconnections). The document Vinod et al. explicitly indicates that the firing step has to be performed at very high temperature (because “low temperature firing at 700° C. is not sufficient to wet and to etch completely the entire thickness of the porous silicon film”). Upon cooling, the Ag/Si layer recrystalizes so as to create the desired ohmic contact.
The step of forming the porous silicon is performed with constant process parameters (i.e., current density). Moreover, the recrystalization process (especially of an alloy like the Ag/Si layer) normally generates a homogeneous structure (for example, see B. Arzamasov, Material Science Edit, Mir Publisher Moscow, English translation 1989, chapter 4.3, page 91, ISBN 5-03-000074-7, the entire disclosure of which is herein incorporated by reference, wherein there is stated that “recrystallization is understood as the nucleation and growth of new grains with a smaller number of structural defect [sic]; recrystallization results in the formation of entirely new, most often equiaxed crystals” and that “as a rule, recrystallized alloy [sic] are homogeneous in their properties and exhibit no anisotropy”). Moreover, the high temperature to which the porous silicon is subject after its formation tends to reduce the superficial porosity (for example, see M. Banerjee et al., “Thermal annealing of porous silicon to develop a quasi monocrystalline structure”, J Mater Sci: Mater Electron (2009) 20:305-311 DOI 10.1007/s10854-008-9725-y, the entire disclosure of which is herein incorporated by reference, wherein there is stated that after a thermal treatment “porous silicon was transformed into quasi monocrystalline porous silicon with a smooth surface and with few voids embedded inside the body”).
Moreover, US-A-2009/0188553 (the entire disclosure of which is herein incorporated by reference) proposes using a porous silicon layer on the front surface to prevent recombination of the generated electric charges. Alternatively, the porous silicon layer may be used to getter impurities of the substrate; in this case, the substrate is annealed to diffuse the impurities into the porous silicon layer, which is then removed. The same document also suggests plating the front contact terminal on an adhesion-promoting porous silicon layer. For this purpose, grooves are made on the front surface; the porous silicon layer is then formed within the grooves, so as to provide an adhesion-promoting surface for the next plating of corresponding buried electrical contacts (with another porous silicon layer that may also be formed on the back surface, for its passivation followed by the opening of windows for contacting the substrate by a metallization layer that is deposited over this passivation layer). In another embodiment, a metallization layer is directly deposited on a porous silicon layer that is formed on the whole back surface; in this case, the front contact terminals are plated on corresponding electrical contact regions, which are obtained by selectively irradiating a photo-catalyst layer on which a hole-scavenger layer is applied. At the end, in a different embodiment the front contact terminals are formed by plating corresponding precursor electrical contacts; the precursor electrical contacts are formed on a porous silicon layer by a screen-printing and etching process. However, these techniques suffer from the same drawbacks pointed out above—i.e., the weakening of the mechanical structure being caused by the grooves (that requires the solar cell to be maintained relatively thick), and the high production costs being caused by the formation of the electrical contact regions or the precursor electrical contacts.
The porous silicon is also used in completely different applications. For example, in WO/2007/104799A1 (the entire disclosure of which is herein incorporated by reference) a porous silicon layer is formed on a substrate to facilitate the raising of leads being formed thereon, so as to obtain corresponding interconnection elements (after the substrate has been removed). For this purpose, the porous silicon layer is configured to allow the peeling of a portion of the leads from the substrate, but at the same time preventing their complete detachment; particularly, the porous silicon layer has a porosity that preferably decreases moving towards its portion to be raised. In any case, the porous silicon layer is relatively thick (for example, at least 2 μm), with a porosity that may also decrease moving inwards the substrate (with the resulting weakening of the substrate that it is not a problem, since it is generally removed after the formation of the raised leads).
In general terms, one or more embodiments are based on the idea of using the porous silicon to anchor the contact terminals on the substrate of the solar cells (or more generally, of the photovoltaic cells). Moreover, one or more embodiments are based on the idea of using a dynamic meniscus for implementing an electrolytic module or an etching module (which electrolytic module and/or etching module may also be used to implement a production line of the photovoltaic cells).
More specifically, an embodiment provides a photovoltaic cell (or solar cell) including a substrate of semiconductor material (for example, silicon). The photovoltaic cell includes a plurality of contact terminals; each contact terminal is arranged on a corresponding contact area of the substrate for collecting electric charges that are generated in the substrate by the light (for example, on a front surface and/or on a back surface of the substrate). For one or more of the contact areas the substrate includes at least one porous semiconductor region (for example, porous silicon), which extends from the contact area into the substrate for anchoring the whole corresponding contact terminal on the substrate. In an embodiment, each porous semiconductor region has a porosity decreasing moving away from the contact area inwards the substrate.
Another embodiment provides an etching module for performing an etching process on a substrate (for example, for processing these photovoltaic cells). The etching module includes an etching head. In turn, the etching head includes a support element having an operative surface. The etching head then includes one or more delivery mouths for delivering an etching solution on the operative surface. The etching head further includes one or more suction mouths (completely surrounding the delivery mouths on the operative surface) for sucking the delivered etching solution; in this way, there is formed a dynamic meniscus on the operative surface when in contact with a corresponding portion of the substrate.
Another embodiment provides an electrolytic module for performing an electrolytic process (for example, an anodization process or a deposition process) on a substrate (for example, for processing these photovoltaic cells). The electrolytic module includes a set of processing heads. In turn, each processing head includes a support element having an operative surface. The processing head then includes one or more delivery mouths for delivering a solution on the operative surface (with the support element that is made at least partially of an electrically conductive material for contacting the solution). The processing head further includes one or more suction mouths (arranged around the delivery mouths on the operative surface) for sucking the delivered solution; in this way, there is formed a dynamic meniscus on the operative surface when in contact with a corresponding portion of the substrate. One of the processing heads is an electrolytic head for providing a dynamic meniscus of an electrolytic solution. The electrolytic module further includes first biasing means for applying a first biasing voltage to the electrolytic solution through the electrolytic head, and second biasing means for applying a second biasing voltage to the substrate.
A further embodiment provides a production line for producing these photovoltaic cells. The production line includes an etching station; in turn, the etching station includes a set of etching modules as above, each one for clearing a corresponding portion of a contact area on each substrate currently in the etching station. In addition or in alternative, the production line also includes an anodization station; in turn, the anodization station includes a set of electrolytic modules as above, each one for forming a corresponding portion of a porous semiconductor region in the contact area of each substrate currently in the anodization station. In addition or in alternative, the production line further includes a deposition station; in turn, the deposition station includes a set of further electrolytic modules as above, each one for forming a corresponding portion of a contact terminal on the contact area of each substrate currently in the deposition station.
A different embodiment provides a process for producing a photovoltaic cell. Particularly, the process includes the step of providing a substrate of semiconductor material, which has a front surface for absorbing the light. At least one front contact terminal is then formed; the contact terminal is arranged on a front contact area of the front surface for collecting electric charges being generated in the substrate by the light. In an embodiment, the front contact area and the front contact terminal have a flat profile. The step of forming at least one front contact terminal includes forming at least one front porous semiconductor region, which extends from the front contact area into the substrate for anchoring the whole front contact terminal on the substrate. The process further includes chemically depositing the front contact terminal.
In an embodiment, the same steps may also be executed to form at least one back contact terminal on a back surface of the substrate (opposite its front surface).